1. Field of the Invention
This invention relates to the fabrication of microminiature tips, and more particularly, to a method of casting a microminiature tip that provides the same with a sharpened end and a desirable surface profile. It also relates to a tip so cast, and a scanning probe microscope incorporating the same.
2. Prior Art
Microminiature tips have many applications in scientific and engineering technology, such as for sensors and in actuators, vacuum microelectronics, microfluidics and microscopy. They are used either to focus current to a point or as a mechanical probe. Such tips traditionally have been very difficult to make in a consistent, reproducible manner. In this connection, for reliable operation in many situations it is important that the tip end be exceedingly sharp.
There is a paper by Marcus which discusses how to produce a sharpened silicon knife blade, "The Oxidation of Shaped Silicon Surfaces", Journal of the Electrochemical Society, Solid-state Science and Technology, June, 1982, pp. 1278-1282. Several publications exist in the field providing some background information on the manufacture of microminiature tips. One such paper is by Stephani, "Fabrication of Densely Packed Sharp Silicon Field Emitters Using Dry Etching Techniques", given at the Second International Conference on Vacuum Microelectronics, July 1989. Another such paper is by Marcus, "Formation of Silicon Tips with &lt;1 nm Radius", Applied Physics Letter, Vol 56, No 3, Jan. 15, 1990, pp. 236-238.
Reliable, sharp tips are particularly needed for scanning probe microscopes, like scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs). The ends of typical scanning probe microscopes either contact or are placed in close proximity to a surface to be imaged. Tips not only must be sharp, they most desirably also have surface contours that facilitate scanning. Insofar as sharpness is concerned, the radius of the tip end of a tip incorporated into either an STM or an AFM should be less than 500 Angstroms. In other words, it is desirable that scanning probe microscope tips terminate in a single atom. Relative to the tip surface profile, it will be recognized that in order to achieve high resolution, it is important that the remainder of the tip not interfere with appropriate positioning of such end.
There are various types of microfabrication processes for making tips by casting in prefabricated silicon molds. One of such processes is described in U.S. Pat. No. 4,916,002, entitled "Microcasting of Microminiature Tips". The process begins with the creation of a pyramidal pit in a silicon wafer by anisotropic etching of the silicon. The pit is then filled with a tip material such as silicon nitride or metal. The wafer is thereafter bonded to a second substrate, and the original substrate which embodies the mold is selectively etched away until the tip is exposed. In this type of process, the ultimate surface contour and sharpness of the resulting tip is determined primarily by the surface contour and sharpness of the mold. One of the disadvantages of this process is lack of reliability in mold contour and sharpness. Another disadvantage is that any deviation in the shape of the pit due to the natural process variations in a manufacturing environment can result in tips which terminate at a blade rather than a point, deleteriously affecting resolution obtainable with the tip.
Microminiature tips can also be made for attachment to the end of a member to form a cantilever. One process of making such a cantilever is illustrated in U.S. Pat. No. 4,943,719, entitled "Microminiature Cantilever Stylus". The process involves forming rectangular silicon posts on a (100) silicon substrate and etching the top of the posts to leave silicon tips at the corners of the remains of each of the silicon posts. Thereafter, a silicon dioxide cantilever with an integral tip is thermally grown over the silicon wafer and the sharp silicon tip. One disadvantage of this process is that a narrow profile on the resulting tip, in the form of a concave surface contour, is not achievable.
The applicant currently has another patent application pending that relates to a method of sharpening silicon tips that are already made. This application was filed on Oct. 10, 1989 and given Ser. No. 07/418,663, abandoned. It only teaches how to sharpen tips made of silicon, not other materials. Moreover, the application neither teaches nor suggests how one could form a tip initially with a desired sharpened end and a desired surface profile. It only teaches how to improve silicon tips that are already made.